Before posting I went back and reread what everyone else said, and sounds like it’s all been covered. I’m posting anyway just hoping to provide a little more thought on what’s already been said.
The Dramatica MC is a perspective, not a character. That said, there are lots of ways to show other throughlines from the MC players POV. Let’s say your MC player is named John. John has a Universe problem that only he is dealing with. Everyone has a Physics problem that they are dealing with, including John. When John is dealing with Universe problems, it is from the MC perspective. When he is dealing with Physics problems, it is from an OS perspective. In that way, you can show OS and MC through one character.
Even so, your story still may have your MC player looking only at his MC problem until the IC shows up.
You shouldn’t have to cram act 1 stuff into act 1. All SP1 material belongs in a Dramatica act 1. It may not all belong in act 1 of a traditional 3 act structure, but those aren’t the same as Dramatica acts.
Also, all SPs take place in order (all SP1s followed by all SP2s), but it doesn’t matter when the audience gets that info as long as at the end they know what’s going on and that everything happened in the right spot. If as soon as the MC and IC characters meet is not a good time to go into OS SP1, wait until a better time. Look at Memento. The audience gets stuff all out of order in that movie.